1. Field of the Invention
The present invention is related to methods and apparatus for producing an image and more especially related to methods and apparatus for producing images of X-ray focal spots by point absorption of radiation.
2. Background of the Prior Art
Aristotle discovered the "pinhole" camera when he noted that the sun's image remains round when it enters a darkened room through a rectangular hole. He was unable to account for this phenomenon, but later researches learned that radiation propagates in straight lines thus allowing a small opening to pass radiation selectively and reproduce an image. The image reproduced by a conventional pinhole camera is topologically similar to (i.e. has the same shape) as the object it represents. The image of a conventional pinhole camera also reproduces the color hue and intensity of the original object.
The topological similarity of the image and object is a result of the size and placement of the pinhole. As the pinhole is made smaller, the image gains detail because smaller portions of the object are discretely mapped onto the image. The size of the image is determined by the pinhole's location between the object and image. If the pinhole is located halfway between the image and the object, then the image will be the same size as the object. If the pinhole is placed otherwise, then the relationship between the object size and the size of the image will vary accordingly to well known laws of geometrical optics. The image will be distorted if the pinhole is not located on the axis normal to the image plane running directly between the object and the image. This distortion is well understood by those skilled in the art of geometrical optics. However distorted, large or small the image may become, it remains topologically equivalent to the object. The particular homeomorphic transformation worked on the image is a function of pinhole placement between the object and image while the resolution of the image is a function of pinhole size.
In contrast to the topological information about the object determined by pinhole location and size, the color hue and intensity of the image formed by a pinhole camera are a function of frequency and power density, respectively, of the radiation passing through the pinhole.
The major drawback to the pinhole camera is the fact that a very small hole must be used to obtain a useable image. This in turn limits the amount of radiation that can be passed to the image in a given time. Pinhole cameras, thus, are very slow and possess very high focal ratios. The focal ratio is the ratio of the diameter of the pinhole to the distance between the back of the pinhole and the front of the image plane. In a typical pinhole camera this ratio will be at least 1 to 100.
This high focal ratio also gives the pinhole camera its major advantage. The higher the focal ratio, the greater the depth of the field of the camera. Thus pinhole cameras have very great depths of field and will reproduce a sharp image of an object over a wide range of distances. This property was used to advantage during the recent skylab space project when a television equipped pinhole camera was used to guide the astronauts in docking their spacecraft.
Another advantage of the pinhole camera is that it can produce an image using a wide range of different types of radiation. A conventional lens system depends on the refraction of radiation to form an image. This works fine for light and frequencies in the infrared and ultra violet that are near light in the electromagnetic spectrum. Unfortunately, it is very difficult to refract higher energy radiation such as X-rays, gamma rays, or particulate radiation such as electrons, protons and neutrons. A pinhole camera, however, produces an image by selective transmission of radiation and thus operates just as well with X-rays or particulate radiation as with light. Because of this quality the pinhole camera principal has found great utility in work with X-rays and neutron radiography.
X-rays are generally produced by impinging high energy electrons on a metal target. The quantum properties of the target's metal atoms determine the frequency of the resulting X-rays. It is normal practice to focus the electron beam onto a small spot on the target tube to give the X-rays a pointlike source. This spot is called the "focal spot" of the X-ray tube and its geometry is of interest to persons skilled in the art of radiology.
Pinhole cameras have long been used to examine the size and shape of X-ray tube focal spots. Unfortunately, this technique has a serious drawback. The intense beam of electrons striking the focal spot produces heat that can damage the X-ray tube. The pinhole camera, as discussed above, requires a long or powerful exposure. The tube must thus be operated at high power levels or for a long period of time, or both, to obtain an adequate image. This method of operation limits the life of the X-ray tube and increases the probability of causing an "observer effect," i.e. of so changing the focal spot during the pinhole camera exposure that the data obtained from the image is not accurate after the picture has been taken.
A more detailed understanding of the role of these minute focal spots and their imaging by pinhole cameras may be had by reading Eric Milne's article The Role of Minute Focal Spots in Roentgenology with Special Reference to Magnification, in the May 1971 issue of CRC CRITICAL REVIEWS IN RADIOLOGICAL SCIENCES, which is herewith incorporated by reference.